A 34-year-old man was found to have leukocytosis and thrombocytosis in 1983. In 1988, his leukocyte count was 10,400/microliter, Hb 16.5g and a platelet was 73 x 10(4)/microliter. A bone marrow examination showed megakaryocyte hyperplasia. Essential thrombocythemia (ET) was diagnosed but no treatment was given. In February 1993, anemia and hepatosplenomegaly developed and cytogenetic study of the peripheral blood demonstrated t(1;7) (q10;p10). Myelofibrosis was diagnosed as by bone marrow biopsy. The patient was treated with blood transfusion, oxymetholone and prednisolone, but without effect. In 1995, acute myeloid leukemia developed, and he died in December 1995 due to septicemia. We report here a case of the ET developed myelofibrosis with t(1;7) (q10;p10) anomaly and acute leukemia.